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LTC1255

of sensitive electrical loads. (Resistor R2, and the diode

D1, provide a direct path for the LTC1255 protection

circuitry to quickly discharge the gate in the event of an

overcurrent condition.)

The RC network, RDELAY and CDELAY, in series with the

drain sense input should be set to trip based on the

expected characteristics of the load after startup, i.e., with

this circuit, it is possible to power a large capacitive load

and still react quickly to an overcurrent condition. The

ramp rate at the output of the switch as it lifts off ground

is approximately:

dV/dt = (VGATE – VTH)/(R1 × C1)

Therefore, the current flowing into the capacitor during

startup is approximately:

ISTARTUP = CLOAD × dV/dt

Using the values shown in Figure 3, the startup current is

less than 100mA and does not false trigger the drain sense

circuitry which is set at 2.7A with a 1ms delay.

Lamp Loads

The in-rush current created by a lamp during turn-on can

be 10 to 20 times greater than the rated operating current.

The circuit shown in Figure 4 shifts the current limit

threshold up by a factor of 11:1 (to 30A) for a short period

of time while the bulb is turned on. The current limit then

drops down to 2.7A after the in-rush current has subsided.

APPLICATIO S I FOR ATIO

DS1

1/2 LTC1255

GND

IN1

VN2222LL

9.1V

12V

470

µF

RSENSE

0.036

Ω

LTC1255 • F04

12V/1A

BULB

10k

100k

0.1

µF

MTP3055EL

Figure 4. Lamp Driver With Delayed Protection

Selecting RDELAY and CDELAY

Figure 5 is a graph of normalized overcurrent shutdown

time versus normalized MOSFET current. This graph is

used to select the two delay components, RDELAY and

CDELAY, which make up a simple RC delay between the

drain sense input and the drain sense resistor.

The Y axis of the graph is normalized to one RC time

constant. The X axis is normalized to the set current.

(The set current is defined as the current required to

develop 100mV across the drain sense resistor.)

Note that the shutdown time is shorter for increasing

levels of MOSFET current. This ensures that the total

energy dissipated by the MOSFET is always within the

bounds established by the manufacturer for safe opera-

tion. (See MOSFET data sheet for further S.O.A.

information.)

NORMALIZED MOSFET CURRENT (1 = SET CURRENT)

0.1

0.01

0.1

100

LTC1255 • F05

Figure 5. Normalized Delay Time vs MOSFET Current

Using a Speed-Up Diode

Another way to reduce the amount of time that the

power MOSFET is in a short-circuit condition is to

“bypass” the delay resistor with a small signal diode as

shown in Figure 6. The diode will engage when the drop

across the drain sense resistor exceeds about 0.7V,

providing a direct path to the sense pin and dramatically

reducing the amount of time the MOSFET is in an

overload condition. The drain sense resistor value is

selected to limit the maximum DC current to 4A.

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